The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The UMTS Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, an E-UTRAN typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 100, commonly referred to as eNodeB. The eNodeB serves an area referred to as a cell 110. An UE entering a cell needs to know the configuration information of that specific cell in order to be able to communicate with the eNodeB serving the cell. Such cell specific cell configuration information is typically broadcasted to all UEs in a cell.
In E-UTRAN, the configuration information of a cell is comprised in the System Information (SI). In order to minimize the resources needed for SI and the power consumption in the UE for reading SI, while still making it possible for the UE to read time critical SI without a significant delay, the SI is grouped in blocks that are broadcasted differently. These blocks are the Master Information Block (MIB) and several System Information Blocks (SIB) of different types, referred to as SIB1, SIB2 etc. The MIB is used to specify which SIBs that are in use in a cell and how they should be accessed. Each SIB contains a field specifying the block type and the actual SI elements, and may optionally also contain an expiration time and a value tag related to the values of the information elements in a SIB. The purpose of the expiration time and the value tag is to avoid an unnecessary re-reading of the SIBs. Examples of SI contained in SIBs are radio resource configuration information common for all UEs (in SIB2) and cell re-selection information common for intra-frequency, inter-frequency and/or inter-RAT cell re-selection (in SIB3).
Changes of SI only occur in specific radio frames. The time between the occurrence of these specific radio frames is referred to as the modification period. Depending on the configuration (e.g. on the length of the paging cycle) the modification period can be as long as 41 seconds. More than one modification period is typically required when the SI is changed. When the network changes parts of the SI, it first notifies the UEs about this change. This may typically be done throughout a first modification period. In the next modification period, the network transmits the updated SI. In order to notify UEs in idle mode as well as connected mode UEs in state CELL_PCH and URA_PCH about a SI change, a paging message is used. Some cell specific SI elements, such as neighbor cell relations, are updated relatively seldom, while other SI elements need to be updated periodically e.g. as a result of interference or traffic measurements.
However, in order to ensure that UEs always have valid SI, the SI elements may only change a few times per hour. The value tag in a SIB is an integer between 0 and 31, which indicates if a change has occurred in the SI elements. UEs may use the SIB value tag, e.g. upon return from out of coverage, to verify if the previously stored SI elements are still valid. In addition to checking the value tag, the UE also considers stored SI to be invalid after three hours from the moment it was successfully confirmed as valid, unless specified otherwise. As the value tag is restricted to an integer between 0 and 31, this implies that it is not safe to change the SI more than 31 times during three hours, i.e. approximately once every 6 minutes in average. Reconfigurations of a cell that require changes in at least one part of the SI is done via modification of the SI. Such a cell reconfiguration, e.g. a reduction of the cell bandwidth in order to lower power consumption, is thus a rather slow and inflexible process.
The number of transmit antennas is not encoded as a bit pattern in the SI, which means that the UEs cannot find out about the number of antennas through the payload of the SI. The RBS will instead choose a transmit format of the broadcast channel (BCH) which corresponds to the actual number of transmit antennas. However, the UEs are not informed about the transmit format of the BCH, and will have to detect it blindly. Once detected, the UEs will thus know how many transmit antennas that the RBS uses, and the UEs will be expecting a certain transmission format of the broadcast channel. A change of the number of transmit antennas is thus not possible without a restart of the cell, which would disturb ongoing traffic.